The present invention relates to an electrophotographic photoconductor (hereinafter also called xe2x80x9ca photoconductorxe2x80x9d). More specifically, the present invention relates to a photoconductor having a photosensitive layer formed on a conductive substrate, the photosensitive layer including charge generation substance, charge transport substance, and a binder resin. Such a photoconductor is useful for printers and copiers employing electrophotographic system.
A photo conductor, having a general structure of a conductive substrate and a photosensitive layer laminated on the substrate, exhibits a photo conductive function. A photoconductor called xe2x80x9can organic photoconductorxe2x80x9d contains organic compounds as functional components serving for charge generation and charge transport. Particularly, a laminated-layer type organic photoconductor, laminating functional layers including a charge generation layer and a charge transport layer, has advantages, such as flexibility in material selection, easy design of performances, high productivity by use of coating process, and superior safety. Therefore, application of such organic photoconductors to various types of copiers and printers has been actively researched in recent years.
In particular, a system that uses hole-transport substance of a distyryl compound having a triphenylamine skeleton and a binder resin of polycarbonate for a hole-transport layer is expected to provide a photoconductor with high responsibility due to high hole mobility of the system. However, there was a problem of abrasion caused by mechanical stresses by image-transfer with light-exposure and by a blade for toner removal.
In recent years, organic photoconductors have remarkably developed in sensitivity and durability against repeated printings by virtue of inventions of charge generation materials and charge transport materials exhibiting excellent characteristics, as well as inventions of resins exhibiting high mechanical strength and favorable compatibility. Nevertheless, the known organic photoconductors are inferior in durability against repeated printings to photoconductors using inorganic materials of selenium and tellurium, as well as to photoconductors using amorphous silicon.
In order to solve the above problem, attempts to improve durability against repeated printings have been made by using polycarbonate with a large viscosity-average molecular weight. For example, the use of bisphenol A polycarbonate resin is disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. S62-160458, and the use of bisphenol Z polycarbonate resin is disclosed in Japanese Unexamined Patent Application Publication (KOKAI) No. H5-165230. However, technology has not yet been established that satisfies requirements for suppressing film-abrasion and for preventing xe2x80x9cfilmingxe2x80x9d, which is caused by the toner attached on the photoconductor surface.
It is an object of the present invention to provide an electrophotographic photoconductor which solves the foregoing problems.
It is a further object of the present invention to provide an electrophotographic photo conductor that exhibits minimal film-abrasion, as well as minimal probability of filming, and thus, high stability under repeated use for a long period of time, while retaining favorable characteristics of an organic photoconductor.
To solve the above problem, an electrophotographic photoconductor according to one aspect of the present invention comprises a conductive substrate and a photosensitive layer on the substrate. The photosensitive layer contains a charge generation substance, a charge transport substance and a binder resin, wherein the binder resin has a dispersion d1, which indicates a range of molecular weight distribution of the resin and is defined by a ratio of the z-average molecular weight Mz to the weight-average molecular weight Mw, i.e., d1=Mz/Mw, of 1.6 or larger in a value converted to polystyrene standard. Moreover, the binder resin has a polydispersity d2, which also indicates a range of molecular weight distribution of the resin and is defined by a ratio of a weight-average molecular weight Mw to a number-average molecular weight Mn, i.e., d2=Mw/Mn, of 2.0 or larger in a value converted to polystyrene standard.
An electrophotographic photoconductor according to another aspect of the present invention comprises a conductive substrate and a photosensitive layer including a charge generation layer and a charge transport layer on the substrate. The photosensitive layer contains a charge generation substance. The charge transport layer contains a charge transport substance and a binder resin, wherein the binder resin has a dispersion d1, which indicates a range of molecular weight distribution of the resin and is defined by a ratio of the z-average molecular weight Mz to the weight-average molecular weight Mw, i.e., d1=Mz/Mw, of 1.6 or larger in a value converted to polystyrene standard. Moreover, the binder resin has a polydispersity d2, which also indicates a range of molecular weight distribution of the resin and is defined by a ratio of a weight-average molecular weight Mw to a number-average molecular weight Mn, i.e., d2=Mw/Mn, of 2.0 or larger in a value converted to polystyrene standard.
In the above two embodiments, the binder resin preferably is prepared so that the dispersion d1 ranges from 1.6 to 3.2 and the polydispersity d2 ranges from 2.0 to 3.7. More preferably, the dispersion d1 ranges from 1.6 to 2.6 and the polydispersity d2 ranges from 2.0 to 3.2. According to another embodiment, the binder resin has a dispersion d1 of from 1.6 to 3.250 and a polydispersity d2 of from 2.0 to 3.800. According to yet another embodiment, the binder resin has a dispersion d1 of from 1.6, 1.7, 1.8, 1.9, or 2.0 to 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, or 3.2 and a polydispersity d2 of from 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, to 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, or 3.7.
According to one embodiment, the charge transport layer and/or photosensitive layer contains only one type of binder resin.
Advantageously, a polycarbonate resin may be used as the binder resin.
According to another embodiment, the photosensitive layer is free of organosilanes.
According to yet another embodiment, one or both of the charge transport layer and the charge generation layer are free of organosilanes.
To solve the problem described earlier, the inventors of the present invention have made numerous studies and reached an idea, while not holding to any one particular theory, that giving the polymer of the binder resin a wide range of molecular weight and large overlapping formed by entanglement of principal chains of the polymer should be effective for improving abrasion resistance and also preventing filming of a photoconductor.
A synthetic polymer material is a collection of various species of molecules having different molecular weights. Mean values of the molecular weight differ each other depending on their calculation methods. There are three mean values of molecular weight: (1) a z-average molecular weight Mz averaged over z-values of each species of molecule, (2) a weight-average molecular weight Mw averaged over total weights of each species of molecule, and (3) a number-average molecular weight Mn simply averaged over molecular weights of each species of molecule. Precise definitions of these averages will be given later by equations (1), (2) and (3).
A collection of molecules consisting of molecules with wide range of molecular weight distribution has large difference between two averages of the three averages mentioned above. Namely, this kind of collection of molecules shows large difference between a z-average molecular weight and a weight-average molecular weight or large difference between a weight-average molecular weight and a number-average molecular weight. d1=Mz/Mw, a ratio of a z-average molecular weight to a weight-average molecular weight, is called dispersion and is an indicator of a range of molecular weight distribution. While, d2=Mw/Mn, a ratio of weight-average molecular weight to number-average molecular weight, is called polydispersity and is another indicator of a range of molecular weight distribution. Thus, the range of molecular weight distribution may be considered in terms of the dispersion d1 or the polydispersity d2.
The average values, a z-average molecular weight Mz, a weight-average molecular weight Mw, and a number-average molecular weight Mn, are defined by the following equations (1), (2) and (3), and actually obtained from a chromatogram of SEC (size exclusion chromatography) using the equations.                     Mz        =                                            ∑                              (                                                      M                    i                    3                                    ⁢                                      N                    i                                                  )                                                    ∑                              (                                                      M                    i                    2                                    ⁢                                      N                    i                                                  )                                              =                                    ∑                              (                                                      H                    i                                    ⁢                                      M                    i                    2                                                  )                                                    ∑                              (                                                      H                    i                                    ⁢                                      M                    i                                                  )                                                                        (        1        )                                Mw        =                                            ∑                              (                                                      w                    i                                    ⁢                                      M                    i                                                  )                                      w                    =                                                    ∑                                  (                                                            M                      i                      2                                        ⁢                                          N                      i                                                        )                                                            ∑                                  (                                                            M                      i                                        ⁢                                          N                      i                                                        )                                                      =                                          ∑                                  (                                                            H                      i                                        ⁢                                          M                      i                                                        )                                                            ∑                                  H                  i                                                                                        (        2        )                                Mn        =                              w                          ∑                              N                i                                              =                                                    ∑                                  (                                                            M                      i                                        ⁢                                          N                      i                                                        )                                                            ∑                                  N                  i                                                      =                                          ∑                                  H                  i                                                            ∑                                  (                                                            H                      i                                        /                                          M                      i                                                        )                                                                                        (        3        )            
where xe2x80x9cwxe2x80x9d represents weight of the sample, xe2x80x9cMxe2x80x9d, a molecular weight, xe2x80x9cNxe2x80x9d, a number of molecules, xe2x80x9cHxe2x80x9d, height of chromatogram, and xe2x80x9cixe2x80x9d represents i-th species of the polymer molecule and corresponds to i-th retention volume in the chromatography.
The inventors of the present invention have made studies on printing durability including abrasion and filming as well as coating characteristic, and have found that excellent printing durability and sensitivity characteristic are obtained by a photoconductor, in which the binder resin of the photosensitive layer of the photoconductor has dispersion d1=Mz/Mw of at least 1.6 or polydispersity d2=Mw/Mn of at least 2.0, where d1 and d2 are values converted to polystyrene standard. Mz is a z-average molecular weight, Mw, a weight-average molecular weight, and Mn, a number-average molecular weight. The present invention has been accomplished based on the finding. The inventors also found in the studies that this favorable effect is significant when polycarbonate is used as a binder resin.
In addition to the above effect, the wide range of molecular weight of the resin used in a photoconductor brings about an advantage in coating characteristic. If only a resin having a large value of a number-average molecular weight is used alone, such problems in coating process arise that viscosity is too large to facilitate coating operation and that the use of large amount of solvent causes excessive cooling of the photoconductor by large heat of vaporization in its drying process down to the temperature under a dew point resulting in dew condensation. Thus, high durability and ease of coating are in a trade-off relationship in conventional photoconductors This problem is solved by a resin having a range of molecular weight distribution larger than certain value according to the present invention.
The photoconductor of the present invention, even in repeated use for a long period of time, holds excellent electrophotographic characteristics, in particular, image reliability and stability in repeated use. The photoconductor of the present invention also may be applied to electrophotographic systems including a laser printer and an electrophotographic platemaking apparatus as well as a copier.
It has been confirmed that an actual electrophotographic system equipped with a photoconductor of the present invention does not cause deterioration of such characteristics as electric potential and sensitivity, even after a long period of time in service. Optical fatigue due to image exposure, mechanical stress due to rollers for charging and transfer and due to blades contacting with the photoconductor for toner removal, and thermal fatigue would cause abrasion and filming of the photoconductor. However, the abrasion and filming are effectively suppressed in a photoconductor of the present invention.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.